Offshore structure having a removable pivot assembly

ABSTRACT

A buoyant tower having a removable pivot assembly is disclosed. The apparatus includes a base provided with means for anchoring it to the bottom of a body of water, an elongated tower that extends upwardly from the base and a buoyancy chamber attached to the tower near its upper end and positioned to remain at least partially submerged beneath the water surface when the tower is installed in a body of water. A primary pivot assembly interconnects the tower and the base and is provided with means for disengaging the same. An auxiliary pivot assembly is included in the tower structure and is situated adjacent the primary pivot assembly. This auxiliary pivot interconnects the tower and base when the primary pivot is disengaged.

United States Patent 1191 Lloyd et al.

[ Oct. 23, 1973 OFFSHORE STRUCTURE HAVING A REMOVABLE PIVOT ASSEMBLY[75] lnventors: James R. Lloyd, Houston, Tex.;

Mark A. Childers, Northridge, Calif.

[73] Assignee: Esso Production Research Company,

Houston, Tex.

221 Filed: Feb. 7, 1972 [21] Appl. No.2 224,005

3,524,323 8/1970 Miller 61/465 Primary Examiner-Milton Buchler AssistantExaminerGregory W. OConnor Att0rneyJamcs A. Reilly et al.

[57] ABSTRACT A buoyant tower having a removable pivot assembly isdisclosed. The apparatus includes a base provided with means foranchoring it to the bottom of a body of water, an elongated tower thatextends upwardly from the base and a buoyancy chamber attached to thetower near its upper end and positioned to remain at least partiallysubmerged beneath the water surface when the tower is installed in abody of water. A primary pivot assembly interconnects the tower and thebase and is provided with means for disengaging the same. An auxiliarypivot assembly is included in the tower structure and is situatedadjacent the primary pivot assembly. This auxiliary pivot interconnectsthe tower and base when the primary pivot is disengaged.

11 Claims, 5 Drawing Figures PAIENTEDHU 23 1915 3.766; 582

SHEET 20F 4 FIG; 4

SHEET 3 OF 4 PATENTEDUCT 23 ms 3166582 sum nor 4 FIG. 5

OFFSHORE STRUCTURE HAVING A REMOVABLE PIVOT ASSEMBLY BACKGROUND OF THEINVENTION l. Field of the Invention This invention relates to apparatuswhich permits removal of the pivot assembly interconnecting a buoyantlysupported tower toa base anchored to a submerged bottom withoutdisrupting normal functioning of the tower.

2. Description of the Prior Art As efforts directed toward explorationfor and production of crude oil and natural gas are extended into deeperwaters, depths will ultimately be reached which will make impracticalthe rigid, bottom-founded platforms presently employed to support suchoperations. One alternative for deep water operations is a buoyantlysupported tower which extends from the water surface to the marinebottom. Such structures normally include a base anchored to thesubmerged bottom, an elongated tower having at least one buoyancychamber near the upper end, and a pivot assembly that connects the towerto the base, permitting the tower to sway in response to environmentalforces.

It is important to the function of a buoyantly supported tower servingto support equipment for the production of crude oil offshore that thepivot assembly, which normally includes a universal joint or ball joint,remain operative for extended periods of time. The pivot assemblies forsuch multiton structures will be massive and will be subjected tocontinuous swaying of 2 to 3 from vertical in any direction as a resultof the action of wind, waves and ocean currents. During storms, towersway may occasionally attain a magnitude of 5 to The difficultiesencountered in lubricating the sliding surfaces of the submerged pivotare substantial and, unless overcome, may substantially shorten itsoperating life. Moreover, the corrosive brine environment also tends toreduce the operating life of the pivot assembly. Because buoyantlysupported towers will frequently be installed in water depths in excessof diver capabilities, maintenance of the underwater pivot assembly inplace will-pose serious difficulties..0n the other hand, retrieval ofthe pivot assembly for maintenance above the water surface hasheretofore required the entire buoyantly supported tower to be detachedfrom the base assembly. It will therefore be apparent that a need existsfor a system which will facilitate maintenance or replacement of thepivot assembly to assure its continued operation in the corrosiveunderwater environment.

SUMMARY OF THE INVENTION The present invention enables the pivotassembly to 1 tower is installed in a body of water. A primary pivotassembly interconnects the tower and base and is provided with means fordisengaging the same. An auxiliary pivot assembly is situated on thetower structure adjacent the primary pivot assembly and pivotallyinterconnects the tower and base when the primary pivot assembly isdisengaged.

The apparatus of the invention permits the load normally carried by theprimary pivot to be transferred to the auxiliary pivot. With thebuoyantly supported tower pivotally connected to the base by theauxiliary pivot, the primary pivot assembly may be removed formaintenance at the surface or replacement. It will therefore be apparentthat the present invention offers significant advantages over systemsavailable heretofore.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic elevation of abuoyantly supported tower which includes one embodiment of a removablepivot assembly constructed in accordance with the invention.

FIG. 2 is an enlarged cutaway elevation view of the apparatus shown inFIG. 1 and depicts in detail the primary and auxiliary pivot assemblies.

FIG. 3 is an enlarged perspective view of an alternative construction ofthe auxiliary pivot assemblies of FIGS. 1 and 2.

FIG. 4 is a schematic elevation of a buoyantly supported tower whichincludes another embodiment of a removable pivot assembly constructed inaccordance with the invention.

FIG. 5 is an enlarged schematic elevation view, partially in section, ofthe removable pivot assembly of FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENTS With respect to FIG. 1 of thedrawing, in which identical numerals have been used to designateidentical structural elements, a base 11 is shown anchored by a numberof pilings 13 to a submerged bottom 15. An elongated tower 17 having asingle buoyancy chamber 19 integrally connected to its upper end isconnected tothe base by a primary pivot, shown as universal joint 21,which permits the tower to sway in response to environmental forces. Anauxiliary pivot assembly designated generally by numeral 24 is alsoshown and is comprised of a spherical bearing surface 23 supported bybase 11 and a spherical skirt 25 attached near the lower end of thetower and configured such that when it is lowered into contact withbearing surface 23 it will mate with and slidably engage the same.

FIGS. 2 and 3 are enlarged views of the removeable pivot assemblyembodied in the buoyantly supported tower depicted in FIG. 1. Sphericalbearing surface 23 and spherical skirt 25 are shown in FIG. 2 ascontinuous surfaces and in FIG. 3 as constructed of a number of curvedstructural members which are joined together to form spherical surfaces.The bearing surfaces may be constructed of steel or a corrosionresistant alloy.

Primary pivot assembly 21 is shown as a universal joint. It will beapparent, however, that other joints, as for example ball joints, mayalso be used with the apparatus of the invention, the primaryrequirement being tion of crude oil and natural gas it can have a weighton the order of 100 tons.

The means for functionally disengaging the primary pivot assembly shownin FIG. 2 of the drawings is a hydraulically actuated system. Anelectric system or other remotely actuatable arrangment of apparatus forcarrying out this function could also readily be employed. Primaryhydraulic cylinders 33 extend between the pivot assembly and the lowerend of tower 17. Two primary hydraulic cylinders are shown in FIG. 2 andare attached to plate 43 by pivots, not shown, which plate is in turnconnected to the primary pivot. Although only two primary cylinders areshown in the drawings, four or more would normally be utilized.Hydraulic fluid is introduced into primary cylinders 33 to extend rods35 into receptacles 36 in tower 17. Introduction of fluid is continuedand pressure is increased until the force generated by the hydrauliccylinders balances the downwardly directed force imposed on the primarypivot assembly by tower 17. In this connection it will be understoodthat in the embodiment of the invention shown in FIGS. 1-3, prior todisengaging the primary pivot the forces acting throughout the structureshould be such that the primary pivot is in compression. It will benoted from FIG. 2 that the position of dogs 39, which normally extendinto slots 41 in the lower end of tower 17 and transmit the weight ofthe tower to plate 43 of the primary pivot assembly, is controlled byupper latching hydraulic cylinders 37. With the weight of the towersupported by the primary cylinders, hydraulic fluid is introduced intothe upper latching cylinders so as to retract dogs 39 from slots 41 inthe lower end of tower structure 17 and thus free the tower structure tomove vertically relative to plate 43. While supported by the primarypistons it will be noted that the tower is free to pivot with respect tothe base via the primary pivot. The load of the tower is subsequentlytransferred from hydraulic cylinders 33 to spherical bearing surface 23by gradually bleeding off the hydraulic pressure within each of theprimary cylinders. These cylinders pivot as they retract, permitting thetower to be lowered onto the auxiliary pivot assembly while supported bythe primary cylinders. With the weight of the structure carried byauxiliary pivot assembly 24 formed by spherical surfaces 23 and 25 theprimary pivot is functionally disengaged, yet sway of the tower can betolerated without interrupting or rendering hazardous further operationsdirected toward physical disconnections and removal of the primarypivot.

The size and surface area of spherical skirt 25 and spherical bearingsurface 23 will be governed to a large extent by the load to betransferred from the primary pivot to the auxiliary pivot. This load maybe determined from the weight of the tower, including any ballast whichmay be provided near the lower end, less the sum of the buoyant forcesgenerated by the submerged buoyancy chamber. In addition to the load,consideration should be given to the anticipated angular rotation of thepivot, The skirt must be sized such that tower sway will not force thelower part of the skirt to strike the base assembly. Similarly clearancemust be allowed between the skirt and the well conductor pipes which areto be situated adjacent to and around the periphery of the base andwhich extend upwardly and parallel to the tower structure. In view ofthe omnidirectional sway of the tower, it will be apparent that aspherical configuration of the bearing surfaces will provide the bestjoint; however, at some sacrifice to efficiency the surfaces could beconstructed somewhat out of round. The construction shown in FIG. 3wherein the bearing surface 23 and skirt 25 are shown as constructed ofa number of curved members joined to form spherical surfaces isparticularly advantageous since it results in a substantial reduction inthe amount of structural material required to fabricate the auxiliarypivot.

A pair of torque pins 29 are shown mounted on the spherical bearingmember 23 and are aligned with this member such that their axes arecoincident with and pass through the center of the pivot assembly. Thesetorque pins are shown in the drawing as integrally constructed as aunitary part of the bearing member but could alternatively be forgedindependently and then journaled or otherwise connected to the bearingsurface. Spherical skirt 25 includes a pair of bearing sleeves 31appropriately aligned and positioned to receive the torque pins on thebearing member when the skirt is lowered onto and concentricallypositioned about the spherical bearing surface. The torque pins andtheir bearing sleeves are an important aspect of the auxiliary pivotassembly depicted in FIGS. 1-3 as they transmit any torque loadsresulting from environmental forces acting on the tower to the base andthereby prevent the tower and any conductor pipes associated therewithfrom rotating with respect to the base.

With the auxiliary pivot assembly interconnecting the tower and base,the primary pivot assembly may be disconnected for removal.Disconnection of the primary pivot is completed by actuation ofhydraulic disconnect cylinders 45 which retract dogs 47 from slots 49 inbase assembly 11. This frees plate 51, which supports primary pivotassembly 21, to slide with respect to base 1 l.

Removal of the pivot is facilitated by a way that extends through thecenter of the tower from the pivot assembly toward the surface. This wayis shown as having a circular cross section and is designated by numeral40 in the drawings. A series of guide rails 53 may suitably be providedwithin way 40 to guide the pivot and prevent it from swaying from sideto side as it ascends and thereby possibly damaging the tower. Liftingof the pivot through the way may, for example, be accomplished bylowering running tool 55 from the surface until an engagement meanssituated at the lower end of the tool engages and locks the running toolto the pivot assembly. This engagement means may include a .l-slot 57 onthe lower end of running tool 55 which receives and locks therewithin acorresponding outwardly extending dog 59 mounted on the upper end of theprimary pivot assembly. Once the running tool and pivot assembly arelocked together, the running tool is withdrawn to the surface via theway thereby raising the pivot assembly to the surface.

The pivot assembly can be refurbished at the surface prior toreplacement or alternatively may be replaced by a new pivot assembly. Ineither event, the running tool is locked to the pivot prior toreplacement and the pivot is lowered downwardly through way 40 with thesame hoist or lifting apparatus used to withdraw the pivot. As the pivotis lowered it will be guided by the rails 53 within the way until thepivot assembly is positioned such that plate 51 is resting atop base 11and disconnect dogs 47 are aligned with the corresponding slots 49 inthe base assembly. Hydraulic disconnect cylinders 45 are then actuatedto extend dogs 47 into slots 49 of the base assembly. With the primarypivot again connected to the base, primary hydraulic cylinders 33 areactuated to extend rods 35 into slots 36 situated near the lower end ofthe tower structure. With the extensible rods positioned within slots36, hydraulic pressure within the primary cylinders is increased,further extending rods 35 and lifting the tower and skirt 25 off bearingsurface 23. Upraising of the tower-iscontinued until slots 41 near thelower end thereof are in register with dogs 39. With the tower and pivotassembly thus aligned, the upper latching cylinders 37 are actuatedhydraulically to extend dogs 39 into slots 41 in the lower end of thetower. The primary hydraulic cylinders 33 may then be bled off so thatthe weight of the tower is once again transferred through dogs 39 toplate 43 of the primary pivot assembly. This in effect re-engages theprimary pivot assembly.

Shown in FIGS. 4 and 5 of the drawings is another embodiment of theapparatus of the invention. As will be noted from FIG. 4, the primarypivot assembly 21 and auxiliary pivot assembly 24 are positioned inseries along the length of tower l7. Retractable bridging bars 61 areemployed to functionally disengage the primary pivot assembly so thatthe loading normally carried by the primary pivot may be shifted to theauxiliary pivot assembly.

The features of the primary and auxiliary pivot assemblies together withthe apparatus for disengaging each pivot assembly are shown in detail inFIG. 5. It will be noted that primary pivot assembly 21 and auxiliarypivot assembly 24 are positioned atop one another. Retractable bridgingbars 61 associated with the primary pivot assembly 63 and associatedwith the auxiliary pivot assembly, are shown in the position they occupywhen extended and retracted. The position of the bridging bars ispreferably controlled hydraulically and appropriate hydraulic gear isexemplified by that shown in cutaway in FIG. 5 in which a cylinder 65 isdepicted, the upper end of which is attached to the lower end of therigid tower. Shown within cylinder 65 is a piston 67 which is rigidlyconnected to one end of bridging bar 63. Introduction of hydraulic fluidinto the cylinder on either side of the piston will either extends orretract the bridging bar. Each piston has a receptacle therein asexemplified by the slot designated by numeral 69. Each such slot isadapted to receive a hydraulically extensible locking pin 70 whichserves to lock the piston and thereby the bridging bar in the extendedposition. During normal operations, bridging bars 63 associated with theauxiliary pivot assembly would be fully extended and locked into framemember 71 by hydraulically extensible locking pins 73 which are receivedwithin slots 75 in the lower ends of bridging bars 63. With the bridgingbars 63 thus extended and locked to frame member 21 and with bridgingbars 61, which are associated with the primary pivot assembly,retracted, the primary pivot interconnects the tower and the base and isthe only pivot which carries any load.

When it is desired to remove the primary pivot assembly the forcesacting on the tower structure must be such that the primary pivotassembly is in tension. The bridging bars 63 associated with theauxiliary pivot may then be freed by releasing hydraulically extensiblelocking pins 70 and 73 and retracted by introducing hydraulic fluid intothe appropriate hydraulic cylinders. The bridging bars 61 associatedwith the primary pivot assembly are extended hydraulically until theupper ends of bridging bars 61 are positioned within apertures 74 inframe member 71. With the bridging bars thus positioned locking pins areextended to anchor each bridging bar at its extremities. The bridgingbars thus rigidly connect the base assembly to frame member 71 wherebythe primary pivot assembly is functionally disengaged and the loadimposed by the buoyantly supported tower acts only through auxiliarypivot assembly 24.

After the primary pivot assembly is functionally disengaged, upperlatching pins 77 and lower latching pins 79 which connect the pivotassembly to the frame member 71 and base assembly 11, respectively, arehydraulically disengaged whereby the primary pivot assembly includingupper and lower slotted plate members 81 and 83 respectively, isslidable with respect to the base assembly and frame member 71. Asubmersible manipulator or similar device may then be employed to slidethe primary pivot assembly out of the structure so that it may be liftedto the surface and either refurbished or structure so that it may belifted to the surface and either refurbished or replaced. Alternativelyone or more anchor piles having sheaves thereon may be employed inconnection with a hoist on a work boat and a length of cable to withdrawthe primary pivot from the structure.

To replace the pivot assembly the refurbished or replacement pivot islowered into position adjacent the base of the tower assembly and bymeans of a submersible manipulator or a pulley in combination withpilings is slid back into place. The upper and lower latching pins 77and 79 may then be extended to rigidly affix the pivot assembly to base11 and frame member 71. Once this is accomplished and with the auxiliarypivot in tension, the bridging bars associated with the auxiliary pivotassembly are extended and those associated with the primary pivotassembly are unlocked and retracted whereby the load imposed by thetower on the pivot again acts on the primary pivot assembly.

What is claimed is:

1. Offshore apparatus comprising:

a. a base provided with means for anchoring the same at the bottom of abody of water;

b. a tower that extends upwardly from said base toward the surface ofsaid body of water;

c. a buoyancy chamber attached to said tower near the upper end andpositioned to remain at least partially beneath the water surface whenthe tower is installed in said body of water;

d. a primary pivot assembly pivotally connecting the lower end of saidtower to said base and serving to transmit a load imposed by said towerto said base, said pivot assembly adapted to be disengaged therefrom;

e. an auxiliary pivot assembly connected to the apparatus adjacent theprimary pivot assembly and adapted to interconnect said tower and saidbase when said primary pivot assembly is disengaged; and

f. means situated on said structure adjacent said pivot assemblies fortransferring the load acting through said primary pivot assembly to saidauxiliary pivot assembly.

2. The apparatus of claim 1 wherein said auxiliary pivot includes aspherical skirt and a spherical bearing surface configured to matetherewithin.

3. The apparatus of claim 2 wherein said tower includes a vertical wayextending therewithin dimensioned to permit said primary pivot assemblyto pass therethrough and provided with means for guiding the said pivotassembly as it moves vertically therewithin.

4. The apparatus of claim 2 wherein said spherical skirt is attached tothe lower end of said tower and said spherical bearing surface isattached to said base.

5. The apparatus of claim 1 wherein said means for transferring saidload to the auxiliary pivot assembly includes a plurality of hydrauliccylinders.

6. The apparatus of claim 2 wherein said auxiliary pivot assemblyincludes a torque pin interconnecting said spherical bearing surface andsaid spherical skirt.

7. The apparatust of claim 1 wherein said auxiliary pivot is comprisedof a universal joint situated in the tower structure in series with saidprimary pivot.

8. The apparatus of claim 1 wherein said auxiliary pivot is comprised ofa ball joint situated in the tower structure in series with said primarypivot.

9. The apparatus of claim 5 wherein a second plurality of hydrauliccylinders are connected to said apparatus and are situated adjacent saidauxiliary pivot assem- 8 bly.

10. A method of removing a pivot assembly from an offshore structurewherein said structure is of the type including a base anchored to thefloor of a body of water, an elongated tower extending upwardly from thebase and having a buoyancy chamber situated near its upper end, andincluding a primary pivot assembly connecting the tower to the base andserving to transmit a load imposed by said tower to said base and anauxiliary pivot assembly for interconnecting said tower and said basewhen said primary pivot assembly is disengaged, comprising transferringthe load acting through said primary pivot assembly to said auxiliarypivot assembly, disengaging said primary pivot assembly from saidstructure without disaseembling same and withdrawing said primary pivotassembly from the structure.

11. The method as defined by claim 10 wherein said primary pivotassembly is withdrawn from the structure through a vertical wayextending vertically through the tower and provided with means forguiding said pivot assembly therethrough.

UNITED STATES PATENT @FHCE WMQATE l Patent No 3,766,582 Dated October23.. 1973 Inventor(s) James R. Lloyd and Mark A. Childers It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

Column 5 line 34, change "position" to --positions-- Column 5, line 43,change "extends" to -extend--.

Column 6, line 21, delete "structure so that it may be lifted to".Celumn 6 line 22, delete "the surface and either refurbish or".

Signed and seated thie 24th day of September 1974 (SEAL) Atteet:

MeCUY M. GIBSON JR", Co MARSHALL DANN Attesting Officer Commissioner ofPatents

1. Offshore apparatus comprising: a. a base provided with means foranchoring the same at the bottom of a body of water; b. a tower thatextends upwardly from said base toward the surface of said body ofwater; c. a buoyancy chamber attached to said tower near the upper endand positioned to remain at least partially beneath the water surfacewhen the tower is installed in said body of water; d. a primary pivotassembly pivotally connecting the lower end of said tower to said baseand serving to transmit a load imposed by said tower to said base, saidpivot assembly adapted to be disengaged therefrom; e. an auxiliary pivotassembly connected to the apparatus adjacent the primary pivot assemblyand adapted to interconnect said tower and said base when said primarypivot assembly is disengaged; and f. means situated on said structureadjacent said pivot assemblies for transferring the load acting throughsaid primary pivot assembly to said auxiliary pivot assembly.
 2. Theapparatus of claim 1 wherein said auxiliary pivot includes a sphericalskirt and a spherical bearing surface configured to mate therewithin. 3.The apparatus of claim 2 wherein said tower includes a vertical wayextending therewithin dimensioned to permit said primary pivot assemblyto pass therethrough and provided with means for guiding the said pivotassembly as it moves vertically therewithin.
 4. The apparatus of claim 2wherein said spherical skirt is attached to the lower end of said towerand said spherical bearing surface is attached to said base.
 5. Theapparatus of claim 1 wherein said means for transferring said load tothe auxiliary pivot assembly includes a plurality of hydrauliccylinders.
 6. The apparatus of claim 2 wherein said auxiliary pivotassembly includes a torque pin interconnecting said spherical bearingsurface and said spherical skirt.
 7. The apparatus of claim 1 whereinsaid auxiliary pivot is comprised of a universal joint situated in thetower structure in series with said primary pivot.
 8. The apparatus ofclaim 1 wherein said auxiliary pivot is comprised of a ball jointsituated in the tower structure in series with said primary pivot. 9.The apparatus of claim 5 wherein a second plurality of hydrauliccylinders are connected to said apparatus and are situated adjacent saidauxiliary pivot assembly.
 10. A method of removing a pivot assembly froman offshore structure wherein said structure is of the type including abase anchored to the floor of a body of water, an elongated towerextending upwardly from the base and having a buoyancy chamber situatednear its upper end, and including a primary pivot assembly connectingthe tower to the base and serving to transmit a load imposed by saidtower to said base and an auxiliary pivot assembly for interconnectingsaid tower and said base when said primary pivot assembly is disengaged,comprising transferring the load acting through said primary pivotassembly to said auxiliary pivot assembly, disengaging said primarypivot assembly from said structure without disassembling same andwithdrawing said primary pivot assembly from the structure.
 11. Themethod as defined by claim 10 wherein said primary pivot assembly iswithdrawn from the structure through a vertical way extending verticallythrough the tower and provided with means for guiding said pivotassembly therethrough.